1. Field of the Invention
The present invention relates to golf balls. In particular, the present invention relates to a golf ball providing an improved shot feel without deterioration in the durability and flight performance.
2. Description of the Background Art
Golf balls generally on the market include solid golf balls and thread-wound golf balls. A solid golf ball is constructed of a solid core and a cover formed over the solid core. The solid core is formed by vulcanizing and molding a rubber composition containing polybutadiene as a main component. A thread-wound golf ball is constructed of a thread-wound core and a cover formed over the core. The thread-wound core is formed by winding a rubber thread around a rubber-based or liquid-based center.
The cover of the solid golf ball is made of hard resin, and thus the solid golf ball is superior in durability and carry (distance) while the solid golf ball is inferior in shot feel. On the other hand, the thread-wound golf ball is excellent in shot feel and controllability while the durability and carry thereof are unsatisfactory.
In recent years, solid golf balls have come to dominate the market since recent solid golf balls provide a soft feel similar to that of conventional thread-wound golf balls with a longer carry or distance retained. Nevertheless, the solid golf balls should still be improved in terms of compromise between the shot feel and the durability and carry.
Two-piece golf balls have been preferred by professional golfers because of the long carry thereof. However, one drawback of the long-carry two-piece golf balls is a poor shot feel. Multi-piece golf balls such as three-piece golf balls are inferior in carry while providing an excellent shot feel since a multi-piece golf ball has a wide range of hardness distribution as compared with a two-piece golf ball.
It is known, as a method of providing a softer shot feel of the golf balls as described above, to soften a core of a golf ball, for example, by increasing a difference between a surface hardness and a center hardness of the core. However, one problem of this method is that the durability of the golf ball tends to deteriorate and the softened core results in a shorter carry.
Various techniques for overcoming the problem above have been disclosed. For example, Japanese Patent Laying-Open No. 60-249979 discloses a golf ball including a core made of a rubber composition containing, per 100 parts by weight of polybutadiene, 25-40 parts by weight of zinc acrylate (calculated as the weight of acrylic acid) and 0.05-0.6 parts by weight of sulfur. This golf ball without organic sulfur compound has a poor rebound property. Moreover, the difference between the surface hardness and the center hardness of the core is not optimized which results in inferior flight performance and shot feel.
U.S. Pat. No. 5,252,652 discloses a multi-layered solid golf ball constituted of a core and a cover, including a base rubber component to which added an unsaturated carboxylic acid metal salt, a zinc salt of any thiophenol, and a crosslinking agent, and having an outer diameter of 37.5-39.5 mm, and its deflection under a load of 100 kg is 3-4.5 mm. This golf ball without sulfur is inferior in durability as well as flight performance and shot feel because the difference between the surface hardness and the center hardness is not optimized.
U.S. Pat. No. 5,645,496 discloses a two-piece golf ball constituted of a core and a cover. The core includes 100 parts by weight of a base rubber to which a co-crosslinking agent and an organic peroxide are added. The center hardness of the core is 40-57 and the surface hardness thereof is 70-95 in terms of JIS-C hardness. The amount of deformation generated between an initial loading of 10 kg and a final loading of 130 kg is 3.0-4.8 mm. Although there is a great difference between the surface hardness and the center hardness of the core of the golf ball, absence of sulfur results in poor durability. Moreover, no organic sulfur compound is added to in the core and accordingly the rebound property is unsatisfactory.
Japanese Patent Laying-Open No. 11-290479 discloses a two-piece solid golf ball constituted of a core and a cover. The center hardness and the surface hardness of the core are respectively 60 or less and 70-95 in terms of JIS-C hardness, and the difference between the center hardness and the surface hardness of the core is 20-40. The amount of deformation of the core under a load from an initial load of 10 kg to a final load of 130 kg is 2.6-3.5 mm. Shore D hardness of the cover is 60 or less and the thickness of the cover is 1.2-2.0 mm. Although the difference between the surface hardness and the center hardness of the core is large, no sulfur is added to the golf ball which accordingly has an inferior durability.
One object of the present invention is thus to provide a golf ball having an improved shot feel without deterioration in the durability and flight performance.
According to one aspect of the present invention, a golf ball is constituted of at least two layers including a core and a cover. The core is formed of a rubber composition including 100 parts by weight of a rubber component, 0.01 to 0.5 parts by weight of sulfur and 0.05 to 5 parts by weight of an organic sulfur compound.
Preferably, an amount of deformation of the core under a load from an initial load of 98 N to a final load of 1274 N is 2.5 to 6.0 mm.
Preferably, a difference determined by subtracting a center hardness of the core from a surface hardness of the core is at least 25 in terms of JIS-C hardness.
Preferably, the core has a center hardness less than 55 in terms of JIS-C hardness.